1. Field of the Invention
This invention relates generally to tube configurations used in heat exchangers and their methods of manufacture.
2. Background Art
In many chemical, electronic, and mechanical systems, thermal energy is transferred from one location to another or from one fluid to another. Heat exchangers allow the transfer of heat from one fluid (liquid or gas) to another fluid. Conventionally, the reasons for transferring heat energy are:
(1) to heat a cooler fluid using a warmer fluid;
(2) to reduce the temperature of a hot fluid by using a cooler fluid;
(3) to boil a liquid using a hotter fluid;
(4) to condense a gas by a cooler fluid; or
(5) to boil a liquid while condensing a hotter fluid in the gaseous state.
Regardless of the function the heat exchanger fulfills, in order to transfer heat, the fluids in thermal contact must be at different temperatures to allow heat to flow from the warmer to the cooler fluid according to the second principle of thermodynamics.
Traditionally, for round tube plate fin heat exchangers there is no direct contact between the two fluids. Heat is transferred from the fluid to the material isolating the two fluids and then to the cooler fluid.
Some of the more common applications of heat exchangers are found in the heating, ventilation, air conditioning and refrigeration (HVACR) systems, electronic equipment, radiators on internal combustion engines, boilers, condensers, and as pre-heaters or coolers in fluid systems.
All air conditioning systems contain at least two heat exchangers—usually an evaporator and a condenser. In each case, the refrigerant flows into the heat exchanger and transfers heat, either gaining or releasing it to the cooling medium. Commonly, the cooling medium is air or water.
A condenser accomplishes this by condensing the refrigerant vapor into a liquid, transferring its phase change (latent) heat to either air or water. In the evaporator, the liquid refrigerant flows into the heat exchanger. Heat flow is reversed as refrigerant evaporates into a vapor and extracts heat required for this phase change from the hotter fluid flowing on the outside of the tubes.
Tubular heat exchangers include those used in an automotive heat exchanger environment, such as a radiator, a heater coil, an air cooler, an intercooler, an evaporator and a condenser for an air-conditioner. For example, a hot fluid flows internally through pipes or tubes while a cooler fluid (such as air) flows over the external surface of the tubes. Thermal energy from the hot internal fluid transfers by conduction to the external surface of the tubes. This energy is then transferred to and absorbed by the external fluid as it flows around the tubes' outer surfaces, thus cooling the internal fluid. In this example, the external surfaces of the tubes act as a surface across which thermal energy is transferred.
Traditionally, longitudinal or radial fins may be positioned in relation to the external surface of the tubes to turbulate the externally flowing fluid, increase the area of the heat transfer surface and thus enhance the heat transfer capacity. One disadvantage, however, is that fins add to material and manufacturing cost, bulk, handling, servicing and overall complexity. Further, they occupy space and therefore reduce the number of tubes that can fit within a given cross sectional area and they collect dust and dirt and may get clogged, thereby diminishing their effectiveness.
Densely configured external fins tend to constrict external fluid flow. This promotes an increase in the pressure drop of the external fluid across the heat transfer surface and may add to heat exchanger costs by requiring more pumping power. In general, expense related to pumping is a function of the pressure drop.
Fin-less, tube heat exchangers are known. See, e.g., U.S. Pat. No. 5,472,047 (Col. 3, lines 12-24). Conventionally, however, they are made of tubes having a relatively large outside diameter. Often, tubes are joined with wires, such as the steel coils found at the back of many residential refrigerators.
The U.S. references identified during a pre-filing investigation were: US 2004/0050540 A1; US 2004/0028940 A1; U.S. Pat. Nos. 5,472,047; 3,326,282; 3,249,154; 3,144,081; 3,111,168; 2,998,228; 2,828,723; 2,749,600; and 1,942,676.
Foreign references identified during a pre-filing investigation were: GB 607,717; GB 644,651; and GB 656,519.